To meet the exhaust emission regulations (NOx and particulate matters (PM) emission regulations) for diesel engines, recent techniques reduce NOx and PM by performing various controls, such as hardware control, operational control, and temperature control of the engine, and also purify exhaust gas using an after-treatment device. As an after-treatment device, a diesel particulate filter (DPF) for collecting PM and an SCR catalyst for purifying NOx are often used.
Using an SCR catalyst, NOx is purified as follows, for instance.    1) Urea is injected at an upstream side of the SCR catalyst in response to the amount of NOx at the upstream side of the SCR, the temperature of the SCR catalyst, the flow rate of the exhaust gas, or the like. The amount of NOx here is measured by a sensor, or estimated.    2) The injected urea is degraded to ammonia (NH3) to be adsorbed on the SCR catalyst.    3) NOx in the exhaust gas is purified by the adsorbed NH3 when passing through the SCR catalyst.
Meanwhile, when the amount of NH3 exceeds an amount that can be adsorbed on the SCR catalyst, NH3 is discharged into the exhaust gas, which is a phenomenon referred to as NH3 slip. When the amount of NH3 adsorption is small, the NOx purification rate decreases. Thus, monitoring the amount of NH3 adsorption is important in determining the purification rate of NOx. However, it is difficult to measure an actual amount of NH3 adsorption, and thus the amount of NH3 adsorption is generally estimated on the basis of e.g. a measurement by an NOx sensor mounted to an exhaust duct. Still, when estimating the amount of NH3 adsorption, an error inevitably occurs due to various causes. For instance, there may be measurement errors or calculation errors of various parameters due to decreases in the accuracy of various sensors, for instance. If subsequent processes are to be continued, errors would be accumulated, which prevents highly accurate control of the amount of urea injection and suitable purification of NOx. As a result, it becomes necessary to initialize the amount of NH3 adsorption.
Patent Document 1 discloses an exhaust purification apparatus of an internal combustion engine which initializes the amount of NH3 adsorption. That is, in the technique for initializing the amount of NH3 adsorption disclosed in Patent Document 1, the amount of reducing-agent adsorption on a NOx catalyst is calculated on the basis of a time-series to balance between supply of the reducing agent to the NOx catalyst upon adding the reducing agent by a reducing-agent adding technique and consumption of the reducing agent in a reducing reaction in the NOx catalyst. Further, the amount of NOx introduced to the NOx catalyst or correlated parameters are calculated. Finally, on the basis of the calculated result, initialization is executed at a determined timing.
Patent Document 1 refers to Japanese Unexamined Patent Application No. 2009-281350